This invention relates generally to a field effect transistor and to a method for protecting such device, and more specifically to a lateral double diffused metal oxide semiconductor (LDMOS) field effect transistor and to a method for protecting that device from a reversed drain voltage.
Semiconductor devices, including discrete devices and integrated circuits, are designed to operate correctly upon the application of specified voltages to the terminals of the devices. Most semiconductor devices would not survive if the voltages applied to the devices were of the reverse polarity from the specified voltages. Some semiconductor devices are designed, for example, to operate with plus 12 volts applied to a given device terminal and most would not survive the application of minus 12 volts to that terminal. Most of such devices would be subjected to a very large and destructive current if the battery connection was accidentally reversed in this manner. Upon the application of the reversed voltage, diodes that are normally reverse biased and able to block an applied voltage of normal polarity would become forward biased and would draw a large forward bias current.
There are a number of applications, however, that require semiconductor devices to survive the accidental application of reversed polarities. For example, in some automotive circuit applications the circuit specification requires the semiconductor devices to survive a xe2x80x9creverse battery condition.xe2x80x9d This would be the condition that would occur if the automobile battery was accidentally connected in reverse.
Solutions that ensure reliability in the event of a reversed voltage application have relied upon adding an additional component in series with the device that is to be protected. For example, in applications requiring such reliability, the typical integrated circuit has been protected by providing an additional protective device or circuit in series with the circuit to be protected. Essentially that amounts to providing a blocking diode in series with the protected circuit. The blocking diode is forward biased during normal operation of the integrated circuit, but is blocking or reverse biased in the event the polarity of the applied voltage is reversed. Such a solution requires one or more additional devices and each of these devices adds an additional unwanted resistance or voltage offset during normal operation of the integrated circuit. For example, if the protective device is a MOSFET (metal oxide semiconductor field effect transistor) in series with the device to be protected, the intrinsic body diode of the MOSFET efficiently blocks any large current that might otherwise flow under reverse battery conditions. Under normal operation of the protected circuit, the on resistance (Rd on) of the protective MOSFET is in series with the resistance of the protected circuit. To make this additional resistance as small as possible (and thus insignificant to the operation of the circuit) the protective MOSFET must be made large. A large additional device is costly and is an inefficient use of available semiconductor area, especially if the protective device is to be integrated with the protected circuit.
In view of the foregoing problem and the lack of a suitable solution, a need exists for an integrated solution that provides the necessary voltage protection in the case of a reversed bias application, that does not add additional voltage offset, and that does not require the use of additional devices.